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When heat is applied to a reaction, what is the result?
What if no heat is applied?
When is the most stable product made?
if heat: 1,4-addition
if none: 1,2-addition
when heat is applied
kinetic control at -80 degrees Celsius
The transition state for 1,2-a ddition has a lower energy than the transition state for 1,4- addition, giving the 1,2- addition a __.
Why is this not surprising?
Because of this lower Ea, 1,2-addition takes place faster.
becaus 1,2-addition results from bromide attack at the more subbed secondary carbon, which bears more of the positive charge because it is better stabilized than the primary carbon
Attach by bromide on the __ is a strongly __ process, so the reverse reaction has a __. At -80 degrees C, few collisions take place with this much energy, and the rate of the reverse reaction is practically __. Under these conditions, the product that is formed faster predominates. Because the kinetics of the reaction determine the results, this situation is called __ of the reaction. The 1,2-product, favored under these conditions, is called the __.
- allylic cation
- large Ea
- kinetic control
- kinetic product
Thermodynamic Control at 40 degrees Celsius
At forty degrees Celsius, a significatnt fraction of molecular collisions have enough energy for __ to occur. Although the 1,2 reaction is still formed aster, it also reverts to the __ faster than the 1,4-product does. At forty degrees Celsius, an equilibrium is set up, and teh relative energy of each species determines its concentration. The 1,4 product is the most __, and it predominates. Since thermodynaics determine the results, this situation is called __ (or __) of the reaction. The 1,4-product, favored under these conditions, is called the __.
- reverse reactions
- allylic cation
- stable species
- thermodynamic control
- equilibrium control
- thermodynamic product
Like __, __ are stabilized by resonance delocalization.
What is the free-radical allylic bromination mechanims?
- allylic cations
- allylic radicals
- Initiation: Formation of radicals
- Propagation (1): the bromine radical abstracts an allylic hydrogen to produce an allylic radical
Propagation (2): the allylic radical in turn reacts with a bromine molecule to form an allyl bromide and a new bromine atom, which continues the chain
Regeneration of Br2: NBS reacts with HBr to regenerate the molecule of bromine used in the allylic bromination step
Stability of Allylic Radicals
Abstraction of allylic hydrogen atom is preferred because the allylic free radical is __. The bond-dissociation enthalpies required to generate several free radicals are compared below. The allyl radical is actually 13 kJ/ mol more stable than the __.
- tertiary butyl radical
Stability of free radicals
Less symmetrical compounds often give mixtures of products resulting from an __: In the product, the double bond can appear at either of the positions it occupies in the __forms of the allylic radical. An allylic shift in a radical reaction is similar to the __.
1,4-addition of an electrophilic reagent such as HBr to a diene.
At higher concentrations, bromine adds across double bonds to give __. In the allylic bromination, bromine __. The key to getting sub is what?
__ are highly reactive and even a small concetration of radicals can produce a fast chain reaction.
Simply adding bromine might raise the concentration too high, resulting in ionic addition of bromine across the double bond. A convenient bromine source for allylic bromination is __.
- saturated dibromides
- substitutes for a Hydrogen atom
- to have a low concentration of bromie, together with light or free radicals to initiate the reaction
Reaction of NBS
The NBS reaction is carried out in a clever way. The allylic compound is dissolved in __, and one equivalent of NBS is added. NBS is __ than CCl4. However, when the reaction is done, the solid succinimide does what?
- 1: Free-radical allylic substitution
- 2: NBS converts the HBr by-product back into BR2
- it rises to the surface because it less dense
If there is an odd number of MOs, they cannot __.
One of the MOs must appear at the __.
It is the __. Electrons in a __ have the same energy as in an isolated p orbital.
The structure of the nonbonding orbital may seem strange because there is __ on the center p orbital (C2). This is the case because pi2 must have __, and the only symmetrical position for one node is where?
be symmetrically divided
middle of the energy elvels, neither bonding nor antibonding
- nonbonding MO
- nonbonding orbital
zero electron density
- one node
- is in the center of the molecule, crossing C2
The electronic configuration of the allyl cation differs from that of the allyl radical; how?
In effect we have removed half an electron from each of C1 and C3, while C2 remains unchanged. This MO picture is consistent with the resonance picture showing the positive charge shared by C1 and C3.
it lacks the unpaired electron in pi2, which has half of its electron density on C1 and half on C3
__ and __ show enhanced reactivity toward nucleophilc displacement reactions by the Sn2 mechanism.
Why is teh rate of Sn2 displacement faster on allylic halides than n-propyl halides?
allylic halides and tosylates
this is because the transition state of the allylic halide receives resonance stabilization through conjugation with the p orbitals of the pi bond, thus lowering the energy of the transition state
The enhanced reactivity of __ and __ makes then attractive as __ for __ reactions. __ are so reactive that they couple with __ and __, a reaction that does not work well with unactivated halides.
- allylic halides
- Allylic halides
- Grignard and organolithium reagents
Alkenes and alkynes with __ add to __ to form __. The __ is called a __ because a ring is formed by the interaction of four pi electrons in the diene, it is caled a __. In effect, the __ converts two pi bonds into two sigma bonds.
The electron movement in this __, with three pairs of electrons moving simultaneously.
- e- withdrawing groups
- conjugated dienes
- six-membered rings
- Diels-Alder reaction
- [4+2] cycloaddition
the D-A reaction
The Diels-Alder reaction is like a __ reaction. The __ is e-rich and the __ is e-poor. Simple dienes are sufficiently __ to be effective dienes for the __. The presence of __ groups, such as alkyl groups or alkoxy groups, may further enhance the reactivity of the diene.
- Diels-Alder reaction
Simple alkenes and alkynes such as ethene and ethyne are poor __, however. A good dienophile generally has one or more __pulling electron density away from teh pi bond. Dienophiles commonly have __ to enhance their Diels-Alder reactivty.
- electron-withdrawing groups
- carbonyl-containing groups or cyano groups
The mechanism of the DA reaction is a __ of six electrons : __ in the diene and __ in the dienophile. For the three pairs of electrons to move simultaneously, the TS must have a geometry that allows what?
- concerted cyclic movement
- overlap of the two end p orbitals of the diene of the dienophile
Three stereochemical features of the DA reaction are controlled by the requirements of the transition state. What are they?
s-cis conformation of the diene: must be in s-cis becuase if s-trans, they are to far apart to overlap and the trans is lower in energy
syn stereochemistry: the DA is a syn addition with respect to both teh diene and dienophiele; the dienophile adds to one face of the diene and the diene adds to one face of the dienophile
the endo rule:
__ that hinder the s-cis conformaiotn react more slowly than butadiene.
Substituents that are on the same side of the diene or dienophile will be __ on the newly formed ring.
When the dienophile has a pi bond in its electron-withdraiwing group, the p orbitals in that e withdrawing group approach one of the central carbon atoms of the diene. This proximity results in __: an overlap of hte p orbitals of the e withdrawing group of the p orbitals of C2 and C3 of the diene. This helps to stabilize the TS
In the bicyclic product, the e-withdrawing substituent occupies the stereochemical position closest to the central atoms of the diene. This posiition is calle dhte __ beccause the substituent seems to be inside the pocket formed by the six-membered ring. This stereochem preference for hte e-withdrawing substituent to appear in the endo position is called the __.
In endo the substituents __.
Even when the diene and dienophile are both __, the DA reaction usually gives a __ rather than a __. We can usually predict the major product by considering how the substituents __the diene and the dienophile in their charge-separated resonance forms. If we arrange the reactants to connect the most negatively charged carbon int eh (electron-rich) diene with the most positively charged carbon in the (electron-poor) dienophile, we can preduct the correct orientation.
point down towards the diene
- unsymmetrically subbed
- single product
- random mixture
An easier way to determine the conformatin of these reactions is to remember that electron donating groups of hte diene and electron withdrawing groups of the dienophile usually bear either a __ or a _ in the products, but not a __.
- 1,4 relationship
- 1,3 relationship